This invention relates to electrical stimulation apparatus and methods for use in stimulating body organs, and more particularly to implantable apparatus and methods for periodic electrical gastric stimulation.
The field of electrical tissue stimulation has recently been expanded to include devices which electrically stimulate the stomach with electrodes implanted in the tissue. These gastric stimulators have been found to successfully combat obesity in certain studies. Medical understanding as to how this treatment functions to reduce obesity is currently incomplete. However, patients successfully treated report achieving normal cycles of hunger and satiation.
An apparatus and treatment method for implementing this therapy was described in U.S. Pat. No. 5,423,872 to Dr. Valerio Cigaina, which is hereby incorporated by reference in its entirety herein. The apparatus described in the Cigaina patent stimulates the stomach antrum pyloricum with trains of stimulating pulses during an interval of about two seconds followed by an “off” interval of about three seconds.
Current pacemaker design incorporates a number of features useful for the type of tissue or organ being stimulated. Pacemakers stimulating cardiac or neurological tissue, for example, may typically contain an accurate, drift-free crystal oscillator to carry out teal-time functions such as pulse generation. In particular, some cardiac pacemakers use a time reference to keep track of the time-of-day with a 24- hour clock in order to log data or to vary pacing parameters during the 24-hour cycle. Similarly, neurological stimulators, such as a neurological stimulator manufactured by Cyberonics, may use the time-of-day as a reference to deliver one or more periods of pulse-train stimulation (typically lasting a few minutes each) to the vagus nerve to treat epilepsy.
The design and operation constraints for a gastric pacemaker, or stimulator, are substantially different from those for a cardiac pacemaker or a neurological pacemaker, for example. With a gastric stimulator for weight loss, size is less of a concern because of the large anatomy associated with obesity. However, a long operating life for an implantable device remains an important feature, given the significantly higher current drain required by this therapy compared to cardiac pacing. Since the implantable pulse generator may be located subcutaneously in the abdominal wall, it is feasible to use a larger device, including a larger, longer-life battery.
Moreover, stomach stimulation may require different levels and cycles of stimulation than that required for cardiac stimulation or nerve stimulation. In a neuromuscular stomach generator, for example, power consumption can be five to seven times higher than for a cardiac pacemaker. Maintaining the proper energy level for stimulation may place energy demands on the life of the battery. The characteristics of entrainment of the stomach tissue may require cycling of the electrical stimulation in more complex schedules than that previously required. Observations of early human implants have shown a surprising increase in the impedance of the electrode tissue interface, from about 700 ohms at time of implant to 1300 ohms after only as much as three months of implants. With constant current and increased impedance, voltage drain on the; battery may be unacceptably high.
Thus, there is a need to optimize the operation of gastric pacemakers, or stimulators, so as to provide a longer life for the device, and hence, a longer duration of therapy without the need for repeated surgical procedures.
It is an advantage to provide an apparatus and method of stimulation wherein voltage or current can be controlled to extend the useful life of a battery used therein.
It is also an advantage of the invention to provide an apparatus and method of stimulation that is able to calculate and store data parameters to improve the levels of stimulation based on operating conditions.
It is a further advantage of the invention to provide a clock function which allows the stimulation cycles of the tissue to be programmed and executed on long term basis.